Image display devices that are worn on the head or the face and used to view images, or in other words, head-mounted displays, are known. With a head-mounted display, an image display unit is disposed for each of the left and right eyes, for example, and an enlarged virtual image of a display image is formed by a virtual image optical system, thereby enabling the user to view an immersive image. Head-mounted displays are extremely popular. If mass production advances further in the future, head-mounted displays may become as common as mobile phones, smartphones, or handheld game consoles, and everyone may come to own their own head-mounted display.
A head-mounted display is configured to cover the user's eyes directly when worn on the head. In other words, a head-mounted display is opaque, thus increasing the sense of immersion when viewing images. A virtual image optical system is used to enlarge and project a display screen, enabling the user to view an image as an enlarged virtual image with a suitable angle of view. In addition, if multichannel audio is reproduced with headphones, it is possible to recreate the sense of being in a movie theater (for example, see Patent Literature 1). On the other hand, see-through head-mounted displays also exist, making it possible to look at the outside surroundings past the image (that is, see through the image) even while the user is wearing the device on his or her head and an image is being displayed (for example, see Patent Literature 2).
Regardless of whether the type is opaque or see-through, a head-mounted display restricts the vision and hearing of the wearing user. For this reason, there are concerns about the user being closed off from the real world and being slower to react to phenomena occurring in the outside world.